Fatigue as a clinical phenomenon is not a new concept. There have been research activities in the 19th and 20th Centuries. Since the early 1960s, ICD-classified diseases, such as chronic fatigue syndrome, neurasthenia or fibromyalgia, are discussed as primarily psychosomatic diseases that are interpreted as somatization disorders. CRF has to be distinguished from chronic fatigue syndrome and other fatigue syndromes, although there are some similarities with respect to phenomenology and theoretical explanation.[4]